spikes or bands: multigroup fingerprints

From: Alberto Fasso' <fasso_at_SLAC.Stanford.EDU>
Date: Mon, 26 Oct 2009 10:16:17 -0700 (PDT)

Dear Mary,

gammas have nothing to do with your question.
Low-energy neutrons are transported in groups, but when a low-energy
neutron is generated in a continuous process (for instance an evaporation
neutron, or a neutron scattered by a hydrogen nucleus), before it is
transported it is assigned the well-defined energy corresponding to that
physical process. At transport time, that energy is converted into the
corresponding group number, and from then on the energy of that neutron
is only known to be somewhere between the lower and the higher limit of a
All normal scoring (USRTRACK, USRBDX, etc.) "sees" low-energy neutrons
only in groups (what you call "bands"), but if you intercept neutrons
with the mgdraw routine, what you see depend on the particular stage of
the code where the interception takes place. You see "bands" (= groups)
if it is during transport (e.g. at the BXDRAW entry) or "spikes"
(= well-defined energies) if it is at production time (e.g. at the
USDRAW entry).


> From: Mary Chin <mary.chin_at_cern.ch>
> Date: Sat, 17 Oct 2009 09:58:33 +0200
> Dear Florian,
> If I may rephrase my question...
> Under the multigroup treatment, an energy spectrum is either not
> continuous or not truly continuous. In FLUKA, is the energy of 1)
> neutrons; 2) secondary gammas sampled discretely (leading to spikes)?
> Does FLUKA apply any smearing (leading to bands)? Or are there
> alternative treatments?
> If the energy is discrete, we would get closer spikes with the 260-
> group library compared to the old library. If the energies are in
> bands/groups, we would get narrower bands compared to the old library.
> Regardless of the multi-group grouping intervals, spikes will remain
> spikes, bands will remain bands. No?
> Poor statistics aren't going to give us tall discrete spikes?
> Temperature - do you mean Doppler broadenning? Ok, to rule out non-
> issues, let us assume zero Kelvin. Scoring - I used mgdraw.
> I could have asked the question without running a simulation at all!
> At this stage I don't think plots or files is necessary. I believe my
> observations are fully predictable and explainable, just that I don't
> know how FLUKA assigns the energy.

Cheers :)

On 16 Oct 2009, at 15:25, Sommerer, Florian wrote:

>> Dear Mary,
>> my guess is that the observed difference is due to the improved gamma
>> production in the 260 gruop library. There are now 42 gamma groups
>> compared to the 22 in the old version. Additionally the data is
>> extracted from the latest evaluated data files. However, your
>> observation could be also due to something else, a problem of
>> statistics
>> maybe.
>> Without knowing precisley what you have simulated (which isotope,
>> which
>> temperature, which scoring,...) it is not possible to answer your
>> question.
>> If you need a more precise answer please send plots of the old and the
>> new spectrum and also all files necessary to reproduce your results.
>> Cheers, Florian
>>> -----Urspr=FCngliche Nachricht-----
>>> Von: owner-fluka-discuss_at_mi.infn.it =
>>> [mailto:owner-fluka-discuss_at_mi.infn.it] Im Auftrag von Mary Chin
>>> Gesendet: Donnerstag, 15. Oktober 2009 12:20
>>> An: fluka-discuss_at_fluka.org
>>> Betreff: spikes or bands: multigroup fingerprints
>>> Hi,
>>> The following is not a problem. Just asking.
>>> If I look at the energy of neutrons simulated with the multigroup =
>>> structure, I find:
>>> - discrete spikes, as opposed to bands. A few years ago (using the =
>>> old/coarse multigroup structure) when I was looking at the energy of =
>>> generated gammas, I found smeared bands, not spikes. What is the =
>>> difference between the two cases? Aren't bands a better
>>> representation =
>>> than spikes?
>>> - the discrete spikes lie *somewhere* between the energies listed in =
>>> Table 10.1 of the manual, not on the border and not really at the
>>> centre =
>>> of the group boundaries. How are the discrete energies chosen?
>>> Many thanks.
>>> :) mary

Alberto Fasso`
SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
Phone: (1 650) 926 4762   Fax: (1 650) 926 3569
Received on Mon Oct 26 2009 - 22:25:27 CET

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